The polymerization of aromatic vinyl monomers including styrene during processing is a matter of concern, because it causes formation of unwanted polymers and results in loss of yield of end product and makes the process un-economical.
In the art use of inhibitors and retarders, and combination thereof to overcome problem of polymerization of styrene has been reported.
The problem of using the inhibitors alone is that these are to be added continuously or at regular interval, because once they are consumed, the polymerization will re-start.
The problem of using the retarders alone is that these are not very effective to reduce polymerization of styrene to a level of substantial inhibition or to the acceptable level of inhibition.
The prior art [U.S. Pat. No. 5,254,760 (US '760)] discloses the polymerization inhibition of vinyl monomers using a combination of nitroxides (i.e. nitroxyl compounds) including 1 oxyl-2,2,6,6, tetramethylpiperidin-4-ol (4HT) and aromatic nitro compounds including dinitro-butylphenol [re abstract, Col. 3, lines 26-32; Col. 4, lines 1-2, 12 of US '760] as the polymerization inhibitor.
The US'760 discloses and teaches use of combination of nitroxides (i.e. nitroxyl) compound and aromatic nitro compound. The US'760 discloses and teaches against the use of either of nitroxides (i.e. nitroxyl) compound or of aromatic nitro compound [Re Col. 5, lines 50-56; Col. 6, lines 10-14 and 42-46; Col. 7, lines 36-41 of US'760].
However, the aromatic nitro compounds including DNBP are to be used in higher amounts and/or are also known for their toxic nature for human exposure [re Col. 1, lines 64-68 of US'760].
Therefore, the industry is aiming for additive composition wherein the aromatic nitro compounds can be avoided. Any effort to avoid consumption of aromatic nitro compounds will lessen the problem of industry.